The present invention relates to pressure sensitive variable resistance devices and in particular relates to pressure sensitive variable resistance switches particularly useful on a keyboard for an electronic musical instrument which actuates the generation or changing of a tone and thereafter causes analog variations in the volume or tonal characteristics in response to the application of a greater or lesser depression of force on the switch.
The generation of musical sounds by electronic means is well known. However, one problem which exists in most electronic instruments is the inability to continuously vary either the volume or the tonal quality of the sound generated. This inability limits the musician's freedom of musical expression. The present invention provides a novel yet simple pressure responsive analog switch having a contact resistance which varies inversely to the amount of pressure applied to depress the analog switch. When used in electronic musical instruments, a plurality of such analog switches may be placed side by side in an elongated fashion to provide a keyboard or one such switch may be used to effect changes in tone by altering the characteristics of one or more tone generating circuits in the musical instrument.
Pressure sensitive analog switches have been known. For example, both in Ruben, U.S. Pat. No. 2,375,178, and Costanzo, U.S. Pat. No. 3,386,067, analog switches are disclosed which sandwich a fibrous or sponge-like layer containing a conductive material between two conductor plates. As the two conductor plates are compressed together the number of electrically conductive paths through the sandwiched layer volume increases, thus decreasing the electrical resistance through that layer. In each of these devices, however, the resistive sandwich layer must be resilient to force the electrodes apart and disconnect most of the conductive paths when the compression force is released. Furthermore, the semiconducting sandwiched layer depends on macroscopic compaction to increase the number of electrical conductive paths between the upper and lower conductor plates. Consequently, the sandwiched layer must have a relatively large thickness. Finally, in such devices the resiliency of the fibrous or sponge-like layer can decrease with use, thus causing a degeneration in the operating characteristics of the switch.
In Mitchell, U.S. Pat. No. 3,806,471, a pressure responsive semiconductor material such as molybdenum disulfide was disclosed, placed between conductor plates to provide an adjustable resistor or transducer. However, Mitchell relies on volume resistance, that is, the resistance through a relatively thick volume of the molybdenum disulfide layer. The present invention on the other hand uses the contact or surface resistance of a very thin layer of molybdenum disulfide. More specifically, Mitchell discloses a molybdenum disulfide volume (thickness) of 0.001 to 1.0 inch using molybdenum disulfide particles in the range of 50 to 600 mesh to provide a high but finite number of three-dimensionally distributed current flow paths through the resistive material. Under compression, the number of current flow paths between the particles in the volume increases, thus causing the resistance to decrease. The semiconductor volume layer is then permanently positioned and attached between two conducting electrodes.
In addition to the above-described functional distinction, the structures disclosed by Mitchell require that the semi-conducting volume be positioned between two electrodes or conductors or otherwise be positioned between a conductor and a nonconductive plate or member so that the semiconducting composition layer does not have any exposed surfaces but rather is in intimate contact with either the insulative plate or the conductors. Such a configuration is fundamentally different from applicant's invention where the semiconducting composition layer must necessarily have at least one contact surface which is not in intimate contact with either a conductor or another semiconducting layer. Such an arrangement facilitates taking advantage of the physical contact resistance over the surface of the composition rather than taking advantage of the surface resistance of the individual particles of material on which Mitchell primarily relies.
The present invention also is exemplified by the use of particle sizes on the order of one micron and layer thickness, preferably less than 0.001 inch. Furthermore, since the variable resistance occurs because of a greater or lesser number of surface contact locations, one surface of the semiconductor layer must be at least initially spaced apart from one of the conducting electrodes or must be in nonintimate contact with the opposing surface, although it may be in touching relationship therewith. Depression of the conducting electrode against the surface of the thin semiconductor layer results in a plurality of contact points being made along the surface. These contact points increase as pressure is applied, thus decreasing the resistance between the conducting plates or contacts on either side of the semiconductor layer. Of course, the surface contact semiconductor layer must be made of any suitable semiconductor material.
A significant advantage of the thin semiconductor layer of the present invention is that the semiconductor material used to form the layer may be combined with a binder and a binder thinner and thereafter sprayed or silk-screened onto the desired surface to form a layer having a thickness as little as one mil or less. Manufacturing costs for both labor and materials are thus greatly decreased.
In addition to the above advantages, the use of molybdenum disulfide to cover the conductive layers effectively protects the surface of the conductor from contact with the air. This alleviates a serious problem which has been attendant with using conductors which slowly corrode when exposed to the air. For example, copper conductors corrode when exposed to the air. This has necessitated the use of expensive silver or other similar and likewise expensive conductive materials. However, when molybdenum disulfide is sprayed or otherwise disposed to cover the conductor, corrosion is greatly reduced which makes possible the use of much less expensive conductor materials such as copper.
Still another significant advantage of the embodiment of the invention where either a conductor and a semiconducting layer surface or two semiconducting layer surfaces are positioned in nonintimate but touching relationship rather than being spaced apart, is that chatter which is inherent in most switches is greatly reduced if not eliminated entirely. Even if the chatter does exist, however, it occurs only when the resistance across the contact of the switch apparatus is so great that the variations in voltage due to the variations in resistance, which cause the chatter will be very small. Consequently, the resultant switch structure in this embodiment is bounceless. Such bounceless switches have significant and substantial commercial applications in the computer industry where there is a constant need for improved bounceless switches of the type disclosed herein. Furthermore, not only is the switch bounceless but it is substantially less expensive than prior bounceless switches.
In Pearlman, et al., U.S. Pat. No. 4,044,642, a touch sensitive resistance device is disclosed for use in musical instruments. However, the device uses a semiconductor material sandwiched between two conductor plates in a manner similar to Ruben and Costanzo. Specifically, Pearlman, et al. uses a resilient material such as foam rubber or foamed synthetic polymeric material which has a particulate material such as graphite dispersed throughout. The switch structure has a foam semiconductor layer and an insulator layer with an orifice therethrough sandwiched between two conductor plates. Thus, when a compression force is applied, the graphite-saturated resilient foam layer deforms into the orifice in the insulator material to initially make electrical contact to thereby switch the musical instrument on. Thereafter, additional compression force causes the resistance between the two conductor plates to decrease in the manner previously described, thereby altering the volume or tonal quality produced.
Because Pearlman, et al. uses a porous foam material there is no problem of air compression in the cavity when the switch is depressed since the air may easily escape and return through the porous resistive material. Furthermore, Pearlman, et al. depends on the physical resiliency of the graphite-impregnated foam material, thus requiring a semiconductor layer of substantially greater thickness than with the present invention. In addition, a degradation in mechanical resiliency of the semiconductor layer also causes a degeneration in switch performance.
It is therefore desirable to provide an analog switch which has a pressure sensitive variable resistance in the ON state but which does not rely upon the resiliency of the semiconductor layer to cause the switch to turn to an OFF state when the compression force is removed. Furthermore, it is desired to provide an analog switch without relying on the volume resistance through a relatively thick semiconductor layer permanently attached between two conductive plates or electrodes.